Water telescope uses gamma rays to track new kind of pulsar

Astronomers might have found a novel way to track down pulsars, the rapidly spinning corpses of massive stars – and the process may have uncovered a new species of pulsar too.

Despite their high energies, pulsars are usually hard to see. They emit powerful beams of radio waves that sweep across space like a lighthouse, but radio telescopes won’t notice these if Earth isn’t in their path.

The other way to see them is via X-rays or heat, but the telescopes used for observations at those wavelengths have narrow fields of view. The supernovae that create pulsars also leave behind beautiful visible nebulae such as the Crab nebula, but only if they are less than a few tens of thousands of years old.

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There should thus be hundreds or thousands of invisible pulsars in the galaxy waiting to be discovered.

Now Tim Linden at Ohio State University in Columbus has a way to find them. The High-Altitude Water Cherenkov (HAWC) observatory – made up of 300 water tanks on a mountaintop in central Mexico – can detect gamma radiation produced by the interaction between charged particles emitted by the pulsars and the gas between stars.

Linden says HAWC can track down previously unknown pulsars, because the gamma-ray emission region covers a bigger part of the sky than radio or X-ray – a few square degrees – and the observatory is designed to look at wider fields than average radio telescopes or orbiting X-ray telescopes. Mapping the location of those gamma-emitting regions can tell the X-ray and radio telescopes where to look to confirm whether a pulsar is present.

Emissions match

To test the idea, Linden and his colleagues observed the sky around two known pulsars to see whether the gamma emissions matched up – and found that they did.

These emissions seemed to also reveal a new class of pulsar, says Linden. Astronomers used to think that a pulsar’s wind of charged particles would be stopped by interstellar gas after a short distance, the same way that the charged particles that stream away from the sun slow down as they run into the interstellar medium. “Everyone thought the region outside that wasn’t very interesting,” Linden says.

But his team spotted gamma rays much further away from one of the pulsars than expected, suggesting that the charged particles could interact with more of the interstellar medium than previously thought.

“This means that the dynamics that produce this emission are new,” says Linden.

His hypothesis is that the pulsar wind interacts with the galaxy’s magnetic field and traps high-energy particles, generating the gamma rays. He says other observatories have seen the gamma emission, but this is the first time anyone has argued that these pulsars might be something different.

Next, Linden plans to request observing time on the European Space Agency’s XMM Newton satellite, which sees in X-rays, to check the regions that HAWC sees for other evidence of unseen pulsars.